Temperature-measuring microwave radiometer apparatus

Abstract

Disclosed are the following three improvements to apparatus employing a microwave radiometer to measure the temperature of a specimen: By situating a low-loss, high-temperature dielectric microwave antenna, which may be composed of low-loss alumina ceramic, within a high-loss-high-temperature antenna sheath, which may be a ceramic composed of an alumina-graphite mixture, a microwave radiometer may be employed to continuously measure the temperature of a material at high temperature, such as molten steel. A Dicke switch replacement for a microwave radiometer comprises a first hybrid ring in which (1) a first combination of first and second input signals thereto derived at a first output port thereof is amplified and forwarded through a switchable phase shifter to a first input port of a second hybrid ring, and (2) a second combination of the first and second input signals thereto derived at a second output port thereof is amplified and forwarded directly to a second input port of the second hybrid ring, resulting in one of the amplified first and second input signals being derived at a first output port of the second hybrid ring and the other of the amplified first and second input signals being derived at a second output port of the second hybrid ring. Switching the switchable phase shifter back-and-forth between 0° and 180° phase shift cyclically reverses the respective output ports of the second hybrid ring at which each of the amplified first and second input signals is derived. A resistance, terminating a coaxial transmission line that couples it to the input of a microwave radiometer, is heated by a heater surrounding it that is energized by an amount of current controlled by the radiometer output. The control is such that the temperature of the resistance reaches a steady state at which it matches the temperature of the specimen. A thermocouple probe in thermal contact with the resistance permits the temperature of the resistance to be indicated by a thermocouple meter.

What is claimed is: 
     
       1. In apparatus employing a microwave radiometer to measure the temperature of a specimen; said apparatus including microwave antenna means which when situated in cooperative relationship with said speciment is responsive to a portion of the microwave interval of the frequency-spectrum distribution of the electromagnetic radiation emitted by said specimen; the improvement wherein said specimen is a given material at high temperature and wherein said microwave antenna means comprises: an antenna sheath; and   a microwave antenna adapted to be disposed within the interior of said antenna sheath, whereby at least a portion of said antenna sheath may be immersed in said given material at high temperature;   said antenna sheath being composed of a high-loss, high-temperature material that is substantially inert to said given material at high temperature so that it can withstand being immersed in said given material at high temperature continuously without substantial damage thereto, and said high loss, high temperature material of which said antenna sheath is composed exhibiting characteristics approaching that of a "black body" for absorbing heat radiation from the surrounding material at high temperature and then reradiating a portion of the absorbed heat radiation as microwave energy into said interior thereof; and   said microwave antenna being composed of a low-loss, high-temperature dielectric material that is substantially inert to the high temperature reradiated into the interior of said antenna sheath and received by said microwave antenna.   
     
     
       2. The apparatus defined in claim 1, wherein: said antenna sheath is in the form of a tube having an opening only at one end thereof; and   said microwave antenna is adapted to be disposed within the interior of said antenna sheath and extend from said opening at one end thereof, whereby said portion of said antenna sheath may be immersed in said given material at high temperature with said opening at one end thereof situated outside of said given material at high temperature.   
     
     
       3. The apparatus defined in claim 1, wherein: said high-loss, high-temperature material of which said antenna sheath is composed is a ceramic material.   
     
     
       4. The apparatus defined in claim 3, wherein: said ceramic material of which said antenna sheath is composed comprises a mixture of alumina and graphite.   
     
     
       5. The apparatus defined in claim 1, wherein: said low-loss, high-temperature dielectric material of which said microwave antenna is composed is a ceramic material.   
     
     
       6. The apparatus defined in claim 5, wherein: said ceramic material of which said microwave antenna is composed comprises low-loss alumina.   
     
     
       7. The apparatus defined in claim 1 wherein: said given material at high temperature is molten steel.   
     
     
       8. In apparatus employing a microwave radiometer to measure the temperature of a specimen; said radiometer including microwave switch means at least responsive in a first state of said microwave switch means to a first microwave signal having an intensity corresponding to the temperature of said specimen and in a second state of said microwave switch means to a second microwave signal having an intensity corresponding to a reference noise signal; the improvement wherein said microwave switch means comprises: a first hybrid circuit including a first input to which said first microwave signal is applied, a second input to which said second microwave signal is applied, a first output from which a first combination of said first and second microwave signals is derived, and a second output from which a second combination of said first and second microwave signals is derived;   a second hybrid circuit including first and second inputs and at least one output;   first means including first amplifier means and a switchable microwave phase shifter for amplifying and forwarding said first combination of first and second microwave signals from said first output of said first hybrid circuit to said first input of said second hybrid circuit, said switchable microwave phase shifter inserting a 0° phase shift in said amplified and forwarded first combination of said first and second microwave signals in a first switch state thereof and inserting a 180° phase shift in said amplified and forwarded first combination of said first and second microwave signals in a second switch state thereof;   second means including second amplifier means for amplifying and forwarding said second combination of first and second microwave signals from said second output of said first hybrid circuit to said second input of said second hybrid circuit;   whereby solely a given amplified one of said first and second microwave signals is derived at said one output of said second hybrid circuit in response to said switchable microwave phase shifter being in its first switch state, and solely the other amplified one of said first and second microwave signals is derived at said one output of said second hybrid circuit in response to said switchable microwave phase shifter being in its second switch state.   
     
     
       9. The apparatus defined in claim 8, wherein: said first hybrid circuit comprises a first hybrid ring including a first input port constituting said first input of said first hybrid circuit, a second input port constituting said second input of said first hybrid circuit, a first output port constituting said first output of said first hybrid circuit, and a second output port constituting said second output of said first hybrid circuit; and   said second hybrid circuit comprises a second hybrid ring including first and second input ports constituting respectively said first and second inputs of said second hybrid circuit, and at least one output port constituting said one output of said second hybrid circuit.   
     
     
       10. The apparatus defined in claim 8, wherein: said switchable microwave phase shifter is responsive to a control signal applied thereto for switching it back-and-forth between its first and second switch states.   
     
     
       11. The apparatus defined in claim 8, wherein: said second hybrid circuit includes first and second outputs;   whereby solely said given amplified one of said first and second microwave signals is derived at said first output of said second hybrid circuit in response to said switchable microwave phase shifter being in its first switch state, solely the other amplified one of said first and second microwave signals is derived at said first output of said second hybrid circuit in response to said switchable microwave phase shifter being in its second switch state, solely the other amplified one of said first and second microwave signals is derived at said second output of said second hybrid circuit in response to said switchable microwave phase shifter being in its first switch state, and solely said given amplified one of said first and second microwave signals is derived at said second output of said second hybrid circuit in response to said switchable microwave phase shifter being in its second switch state.   
     
     
       12. The apparatus defined in claim 11, wherein: said switchable microwave phase shifter is responsive to a control signal applied thereto for switching it back-and-forth between its first and second switch states.   
     
     
       13. The apparatus defined in claim 11, wherein: said radiometer includes means for applying said control signal to said switchable microwave phase shifter, a first broadband receiver for receiving the output from said first output of said second hybrid circuit as an input thereto, a second broadband receiver for receiving the output from said second output of said second hybrid circuit as an input thereto, a first rectifier for rectifying the said first broadband receiver, a second rectifier for rectifying the output of said second broadband receiver, and means for deriving the difference in signal level between the respective outputs of said first and second rectifiers.   
     
     
       14. The apparatus defined in claim 9, wherein: said first combination of first and second microwave signals from said first output of said first hybrid circuit is applied to an input of said first amplifier means, an output of said first amplifier means is applied to an input of said switchable microwave phase shifter, and an output of said switchable microwave phase shifter is forwarded to said first input of said second hybrid circuit.